Enhanced Thermal Stability on Multi-Metal Filled Cermet Based Spectrally Selective Solar Absorbers

ABSTRACT

A spectrally selective solar absorber is described and comprises a substrate, double cermet layers comprising multi-metal nanoparticles embedded in a dielectrics matrix, and double antireflection layers deposited on cermet layers. The tungsten or titanium or tantalum infrared reflector layer suppressing the diffusion of substrate elements and multi-metal nanoparticles in the cermet are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. §371 national stage application ofPCT/US2015/052952 filed Sep. 25, 2015, and entitled “Enhanced ThermalStability on Multi-Metal Filled Cermet Based Spectrally Selective SolarAbsorbers,” which This patent application claims priority to andincorporates in its entirety U.S. Provisional Patent Application62/072,124 filed Oct. 29, 2014, and entitled “Enhanced Thermal Stabilityon Multi-Metal Filled Cermet Based Spectrally Selective SolarAbsorbers,” filed Oct. 29, 2014 each of which are hereby incorporatedherein by reference in their entirety for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This work was partially supported by “Concentrated Solar ThermoelectricPower (CSP)”, DOE SunShot CSP grant, under award number DE-EE0005806 and“Solid State Solar-Thermal Energy Conversion Center (S³TEC),” an EnergyFrontier Research Center funded by the U.S. Department of Energy, Officeof Science, Office of Basic Energy Science under award numberDE-SC0001299/DE-FG02-09ER46577 (GC and ZFR).

BACKGROUND Background of the Technology

Solar thermal technologies such as solar hot water and concentratedsolar power trough systems employ spectrally-selective solar absorbers.These solar absorbers are designed to efficiently absorb the sunlightwhile suppressing re-emission of infrared radiation at elevatedtemperatures.

BRIEF SUMMARY OF THE DISCLOSURE

In an embodiment, a method of fabricating solar absorbers comprising:disposing a first layer in contact with a substrate; disposing a secondlayer in contact with the first layer; disposing a third layer incontact with the second layer; disposing a fourth layer in contact withthe third layer; and disposing a fifth layer in contact with the fourthlayer, wherein disposing the fifth layer forms a solar absorbercomprising an absorbance within a first predetermined range and anemittance within a second predetermined range.

In an embodiment, a solar absorber comprising: a reflector layerdisposed in contact with a substrate; a first cermet layer disposed incontact with the reflector layer; a second cermet layer disposed incontact with the first cermet layer; and at least two anti-reflectivecoating (ARC) layers, wherein at least one ARC layer is disposed incontact with the second cermet layer.

In an embodiment, a solar absorber comprising: a reflector layerdisposed in contact with a substrate; a first cermet layer disposed incontact with the reflector layer, wherein the reflector layer comprisesat least one of at least tungsten (W) or nickel (Ni).

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the exemplary embodiments disclosedherein, reference will now be made to the accompanying drawings inwhich:

FIG. 1 is a schematic of a spectrally selective solar absorberconfiguration according to certain embodiments of the presentdisclosure.

FIG. 2 illustrates the bidirectional reflectance spectra of solarabsorbers before and after annealing that were fabricated according tocertain embodiments of the present disclosure.

FIGS. 3A and 3B illustrate the surface roughness of a solar absorberupon annealing where the solar absorber was fabricated according tocertain embodiments of the present disclosure.

FIGS. 4A-4D are AFM images illustrating the morphology change of asingle cermet layer before and after annealing according to certainembodiments of the present disclosure.

FIG. 5 illustrates XRD patterns for pristine and annealed cermetcoatings where the coatings were fabricated according to certainembodiments of the present disclosure.

FIG. 6 is a chart of Raman spectra of cermet coatings before and afterannealing where the coatings were fabricated according to certainembodiments of the present disclosure.

FIG. 7 illustrates the spectral bidirectional reflectance response ofsolar absorbers that were fabricated according to certain embodiments ofthe present disclosure.

FIG. 8 illustrates XRD patterns of solar absorbers fabricated accordingto certain embodiments of the present disclosure.

FIG. 9 illustrates the experimental set up and results for steady statecalorimetric measurements of samples fabricated according to certainembodiments of the present disclosure.

FIG. 10 illustrates the spectral properties of a plurality of solarabsorbers that were fabricated according to embodiments of the presentdisclosure.

DISCUSSION OF DISCLOSED EXEMPLARY EMBODIMENTS

The paper by F. Cao, D. Kraemer, G. Chen, and Z. Ren, entitled EnhancedThermal Stability of W—Ni—Al2O3 Cermet-based Spectarlly Selective SolarAbsorbers with W Infrared Reflector, is incorporated by this referencein its entirety.

Efforts for the development of thermally stable selective solarabsorbers may focus on spectral selectivity and thermal stability forhigh temperature applications. Selective solar absorbers, which may bereferred to as “solar absorbers” herein, were fabricated according tocertain embodiments of the present disclosure and may be based on twocermet layers and were fabricated using a magnetron sputtering techniqueon mechanically polished stainless steel substrates. Cermets arecomposite materials comprising metallic and ceramic materials that maytherefore comprise desirable properties of both ceramics and metals. Forexample, cermets may be resistant (to loss of properties anddeformation) to high temperatures like a ceramic, and may be able toundergo plastic deformation like metallic materials. Cermets may be usedin both electronic and mechanical applications including in solarapplications and for cutting and machining tools that may alsoexperience high temperature. Reflector layers provide solar reflectanceby reflecting wavelengths in various wavelength ranges, including thevisible, infrared, and ultraviolet ranges, in order to reduce the heattransferred to the surface of an apparatus employing the reflectorlayer. In some embodiments herein, infrared reflector layers may beemployed in solar absorbers. Wavelength ranges may comprise infraredwavelengths above 700 nm (10⁻⁹ m) to about 1 mm, visible wavelengths mayrange from about 400 nm to about 700 nm, ultraviolet light wavelengthsmay range from less than 400 nm (e.g., shorter wavelengths than visiblelight) to about 10 nm, x-rays may range from less than about 10 nm(e.g., shorter than ultraviolet light) to about 10 pm (picometers, 10⁻¹²m), and gamma rays may be less than about 10 pm, that is, shorter thanx-ray wavelengths.

In various contexts and applications, the emittance (emissivity) of asurface may be considered because a low emittance may indicate that thesolar absorber wastes less energy through emitting thermal radiationthan materials with a high emittance. The same principle may apply, forexample, in insulation applications where it may be desirable for awindow to retain heat using a coating or a film. In an embodiment, anoperating temperature where selective solar absorbers may be desired isfrom about 500 to about 600° C. Nickel and tungsten were employed incertain embodiments for the infrared reflector layer in selectivethermal absorbers discussed herein, the results of those experiments arediscussed herein, including one in which a stable solar absorptance ofabout 0.90 and total hemispherical emittance of 0.15 at 500° C. wasobtained using tungsten as the infrared reflector layer. While theinfrared reflector layer may be referred to in some embodiments as “alayer,” the infrared reflector layer may be a plurality of individual(separate) layers which may be of the same or differing layer typesand/or varying thicknesses, or combinations or the same type of materialand different types of material with the same or varying thicknessesdepending upon the embodiment.

In one embodiment, a spectrally selective solar absorber comprises asubstrate (stainless steel, tantalum, titanium, copper, aluminum,nickel, silicon, quartz, and combinations thereof), an infraredreflector layer or bonding layer (tungsten, tantalum, titanium, nickel,silver, gold, aluminum, and combinations thereof), a first and a secondcermet layer which may comprise multi-metal nanoparticles in dielectricmatrix and two anti-reflection coatings. The term “selective” may beused to describe the manner in which the solar absorber is fabricated sothat the solar absorber provides an absorbance within a firstpredetermined wavelength range and an emittance within a secondpredetermined wavelength range. Herein, a “cermet layer,” s acombination of two or more metals and a ceramic, and in someembodiments, a combination of at least two layers cermet1 (“C1”) andcermet 2 (“C2”) may be employed in a solar absorber, where each of C1and C1 comprises a combination of any two or more metals, including butnot limited to Nickel (Ni), Cobalt (Co), Iron (Fe), Tungsten (W),Tantalum (Ta), Titanium (Ti), Molybdenum (Mo), Chromium (Cr), Vanadium(V), Niobium (Nb), Zirconium (Zr), and at least one of Al₂O₃, SiO₂,ZrO₂, Ta₂O₅, AlN, or other dielectric materials as appropriate for theend application's desired absorbance and emittance ranges.

The introduction of multi-metal nanoparticles in cermet, as discussed incertain embodiments of the present disclosure, as compared to the use ofsolely single-metal nanoparticles provides additional tuning parameters(e.g., the metal/ceramic concentrations and different componentselections) with which to tailor the optical properties of the cermetabsorption layers. The anti-reflection coatings (“ARC”), also referredto as “ARC layers” discussed herein may comprise Al₂O₃, MgO, TiO₂, V₂O₅,Ta₂O₅, ZrO₂, SiO₂, and other oxide layers that may be appropriate forvarious desired ranges of emittance and reflectance in solar absorbers.In some embodiments, the stable infrared reflector layer suppresses thediffusion of the substrate elements into the cermet layer and results inenhanced thermal stability of the solar absorber at elevatedtemperature. The metal infrared reflector layer also improves to someextent the spectral selectivity of the solar absorber due to its lowinfrared emittance.

Introduction

Sunlight may be converted into a useful terrestrial heat source byemploying sunlight absorbing surfaces in the form of solar absorbers.Solar absorbers may be employed in solar thermal systems such as solarhot water systems and concentrated solar power (CSP) trough systems, aswell as in emerging technologies such as solar thermoelectric, solarthermo-photovoltaic, and solar thermionic generators. The solar thermalreceiver efficiency may depend on the optical properties of the solarabsorber. To maximize the efficiency of a solar absorber, it may bedesirable for a solar absorber to comprise a near-blackbody absorptance(α) in the solar spectrum range while retaining a low emittance (∈) inthe infrared (IR) range, and be thermally stable at their operationaltemperatures. The solar absorbers discussed herein may be employed inprocesses, methods, and products to convert received wavelengths intoenergy sources.

Discussed herein is a spectrally-selective solar absorber (“solarabsorber”) and methods of fabricating the solar absorber comprising alayer disposed between a substrate and an absorber coating thatdemonstrates a long-term stability at high temperatures (T>400° C.) aswell as a stable solar absorptance of about 0.90 and a hemisphericalemittance of 0.15. As used herein, a “spectrally selective” solarabsorber may be defined by the range of wavelengths it is designed toreflect and/or absorb.

As for mid-temperature (about 100° C.<T<about 400° C.) andhigh-temperature (T>about 400° C.) applications, cermet-based coatingsmay be employed and comprise ceramic metallic composites which may begood candidates for inclusion in the solar absorber due to their highsolar absorptance, low emittance, and good thermal stability. Thesedesirable properties may be attributed to the high temperature stableceramic host. Cermet-based spectrally selective solar absorbers maypresent and be employed as single, double, and triple cermet layers. Thethin cermet layer is typically in contact with a metallic surface forhigh solar absorptance that is transparent to IR radiation. Theabsorption of solar radiation in the cermet layer may be due tointerbank transitions in the metal and small particle plasmonicresonances.

A “graded metal volume fraction” is the term used herein to describe acombination of two or more cermet layers comprising different metalvolume fractions (weight of metallic/(weight of metallic+ceramiccombined)). The graded metal volume fraction between and within thecermet layers gives it a gradual increase in the refractive index fromsurface to the substrate, which reduces reflection compared with singlecermet layer absorbers that often use black metals such as black chrome,black nickel, or black tungsten as their metal fillers. Solar absorbersfabricated according to embodiments of the present disclosure thatcomprise cermet multilayers (C1 and C2 in this example) with differentmetal volume fractions introduces a stepwise change in the refractiveindex that may result in a low reflection of visible light due tointerference effects.

In some embodiments, additional anti-reflection coatings may be appliedto the solar absorbers to further reduce reflection losses.Consequently, cermet-based solar absorbers have a tunable parameterspace (range) based upon their constituents, coating thicknesses,particle concentration, size, shape, and orientation to optimize theirspectral selectivity. Various combinations of host materials such asAl₂O₃, AlN, and SiO₂ with metallic filler atoms such as Ni, Co, Ti, Mo,W, Pt, Stainless steel (SS), Cu, Ag, Au have been investigated in termsof their respective effectiveness for the optical performance andthermal stability of the cermet surfaces. These combinations of hostmaterials have ceramic host materials in common that possess hightemperature stability, and are therefore complimentary. The metal filleratoms may be chosen for their high melting point and their resistance toboth nitriding and oxidation, in order to enhance and ensure thermalstability.

In an embodiment, in the case of solar absorbers with mid-temperatureapplications, the cermet layers may be deposited on metal substratessuch as polished aluminum or copper due to their low IR emittance andhigh thermal conductivity. In an embodiment, a diffusion barrier betweenthe substrate and the cermet layer was introduced with a spontaneouslyformed Fe₂O₃ layer by annealing the stainless steel substrate at 500° C.in air. However, the surface roughness of the substrate changes whenforming an Fe₂O₃ layer, which eventually affects the surface roughnessof solar absorber and then increases the emittance. Also, the Fe₂O₃layer on the back side of the stainless steel may introduce anotherthermal resistance layer in a solar absorber, which will decrease theheat transport efficiency from the absorber to the thermal system.Surface smoothness may be a desirable property in solar absorbers, sothe impact of annealing was evaluated and is discussed herein.

The embodiments herein discuss depositing, for example, a nickel (Ni) ortungsten (W) layer that may be referred to as an inter-reflector (IR)layer onto a mechanically polished substrate that may comprise stainlesssteel. Depending upon the embodiment and the substrate materialemployed, the substrate may not be polished. This IR layer may act noteonly to bond the substrate to other layers but also as a diffusionbarrier and as a low IR emittance coating to improve spectralselectivity. The performance of the metal IR reflector layer with adouble-layer cermet structure and two antireflection coatings (ARCs) isdiscussed herein. In contrast to cermet structures that may be filledwith particles of one metal type, the cermet layers based on an Al₂O₃ceramic host material may be filled with high temperature stable Ni—Walloy prepared by co-sputtering. Therefore, the cermet layers may eachcomprise not only the metal volume fraction in each cermet layer butalso the volume fraction of the individual constituent which may beadjusted to tailor the optical properties of a solar absorber dependingupon the end application, subsequent processing, or customerspecifications.

In one example experiment, a plurality of individual layers of the solarabsorbers were deposited using a magnetron sputtering technique. Thespectral bidirectional reflectance responses of the fabricated solarabsorbers were measured at room temperature before and after annealingat 600° C. for 7 days. The solar absorptance and total hemisphericalemittance were measured at elevated temperatures of up to 500° C.

In an embodiment, the spectrally-selective solar absorbers may bedeposited in contact with a substrate, for example, a mechanicallypolished stainless steel substrate. The deposition may be performedusing a commercial magnetron sputtering equipment (AJA international,Inc.). For the thickness measurement of the C1 and C2 layers, thematerials may be simultaneously deposited on Si wafer substrates partlycovered by a mask. Prior to the deposition process, the chamber may beevacuated to lower than 4×10⁻⁷ Torr. The deposition targets are highpurity nickel (99.999%, 2″ Dia.), tungsten (99.95%, 3″ Dia.), Al₂O₃(99.98%, 2″ Dia.), and SiO₂ (99.995%, 3″ Dia.). DC power is supplied tothe metal targets (Ni, W) to deposit the metal layer and for the metalparticle. The dielectric layer is deposited by RF magnetron sputtering.Co-sputtering may be employed to deposit more or one dielectric layers,such as the C1 and C2 layers. The metal fill fractions of the cermetlayers may be controlled by independent input power control to thecorresponding targets. The complete deposition process may be performedin an argon plasma environment at a pressure of 3 mTorr. The detailedpreparation parameters are summarized in Table 1 herein.

Regarding the thermal stability, the solar absorbers fabricatedaccording to embodiments of the present disclosure are characterized interms of their phase, morphology, and optical properties both before andafter annealing the samples at 600° C. for 7 days at a vacuum pressureof about 5×10⁻³ Torr using a tubular furnace. The X-ray diffraction(XRD) patterns were obtained using a PANalytical multipurposediffractometer with an X'Celerator detector and Cu Kα radiation(λ=1.54056 Å) operating at 45 kV and 40 mA. Raman scattering spectrameasurements were carried out on a T64000 Raman system (Horiba JobinYvon) at room temperature. The excitation source is the 514 nm laserline of an air cooled Ar-ion laser.

The thickness of the cermet films were measured with an Alpha-step 200Profilometer (Tencor). The growth rates of metal and dielectric layerscomprising the cermet layers (films) were measured by a quartz crystalmonitor equipped in the sputtering system. The morphology and roughnessof the films were measured with a Veeco Dimensions 3000 Atomic ForceMicroscope (AFM). The spectral bidirectional reflectance was measured atroom temperature with a Spectrophotometer by Varian (Cary 500i, angle ofincidence 8°, absolute spectral reflectance accessory) covering thewavelength range of 0.3-1.8 μm, and with an FT-IR Spectrometer by ThermoScientific (Nicolet 6700, angle of incidence 12°) covering thewavelength range of 1.8-20 μm. The latter (relative measurement)requires a reference with known spectral reflectance which is chosen tobe a specular gold mirror (Thorlabs).

FIG. 1 is a schematic of a spectrally selective solar absorberconfiguration according to certain embodiments of the presentdisclosure. It is to be appreciated that, while different patterns areused to distinguish the layers, these indications are not necessarilyindicative of differences in the layers that are visible to the nakedeye, and it is also to be understood that the relative thickness oflayers may vary between embodiments. The spectrally selective solarabsorbers fabricated according to certain embodiments of the presentdisclosure for mid- and high-temperature applications are based on adouble cermet layer configuration with two ARC layers and a metal layerwith high IR reflectance as diffusion barrier. The two ARC layers ARC1and ARC2 may also be Al₂O₃ and SiO₂ thin films, respectively.

In alternate embodiments, the ARC1 layer may comprise MgO, TiO₂, V₂O₃,ZrO, or combinations thereof. In order to investigate the effect of theARC layers, the solar absorber multilayer structures were fabricatedaccording to certain embodiments of the present disclosure withtungsten, optically thick nickel, or very thin nickel layer as and IRreflector or bonding layer. The detailed parameters are summarized inTable 1. In the embodiment in Table 1, the substrate may comprise ametal layer, for example, nickel having a DC power density of 12.3 W/cm²or tungsten having a DC power density of 2.2 W/cm² for tungsten. The C1layer may comprise W+Ni+Al₂O₃ with a DC power density of 0.33 W/cm² fortungsten and 0.99 W/cm² for nickel, and a RF power density of 9.9 W/cm²for Al₂O₃. The cermet2 layer may comprise W+Ni+Al₂O₃ with a DC powerdensity of 0.26 W/cm² for tungsten, and 0.74 W/cm² for nickel, and a RFpower density of 9.9 W/cm² for Al₂O₃. The ARC1 layer may comprise Al₂O₃with a RF power density of 9.9 W/cm² and the ARC2 layer may compriseSiO₂ with a RF power density of 4.4 W/cm².

TABLE 1 Substrate Bonding layer/IR C1 thickness C2 thickness ARC1 ARC2Sample material layer thickness/type (nm) (nm) (nm) (nm) C1 SS  10 nm Ni180  N/A N/A N/A C2 SS  10 nm Ni N/A 28 25 55 S-SS SS  10 nm Ni 11 28 2555 S-Ni/SS SS 300 nm Ni 11 28 25 55 S-W/SS SS 300 nm W 11 28 25 55S-W/SS-2 SS 200 nm W 11 28 25 55 S-W/SS-3 SS 100 nm W 11 28 25 55S-W/SS-4 SS  50 nm W 11 28 25 55 S-W/SS-5 SS  10 nm W 11 28 25 55

The multilayer stack that makes up the spectrally selective solarabsorbers fabricated according to certain embodiments of the presentdisclosure may comprise one bonding or IR reflector layer, double cermetabsorption layers and double ARC layers which further reduce reflectionin the visible range. In some embodiments, multiple IR-reflector layersof the same or differing compositions and/or concentrations (metalfractions) may be used in different arrangements in a solar absorber.The use of mechanically polished stainless steel as the substrate mayprovide high temperature stability and may be cost-effective, which canpromote large scale deployment as a potential solar absorber candidatein high temperature solar receivers. It has been shown that elementaldiffusion of iron and carbon from a stainless steel into the cermetlayer can be detrimental for the optical properties, and a diffusionbarrier may be employed to combat this diffusion. Thus, the thermalstability of optimized coatings was evaluated on stainless steel with a10 nm thin nickel bonding layer which may act as a diffusion barrier(the S-SS sample). Details about multilayer stack composition andpreparation parameters are summarized in Table 1 above.

FIG. 2 illustrates the bidirectional reflectance spectra of the pristine(where “pristine” is the term used to describe a condition beforeannealing) and annealed solar absorbers fabricated according to certainembodiments of the present disclosure. The reflectance of the pristinesample is close to zero in the visible range, which is expected for adouble-cermet-absorption-layer combined with a double-ARC-layer due tothe intrinsic absorption of the double-cermet layer and the reflectancereducing interference effects. The sharp transition wavelength rangefrom low reflectance to high reflectance appears to be from about 1 toabout 3 μm, which can result in promising spectral selectivity even athigh temperatures. However, the degraded optical properties of the solarabsorber upon annealing at 600° C. for 7 days show a detrimental effecton spectral selectivity. The spectral reflectance below about 1.1 μmincreases while it decreases above about 1.1 μm which results in abroadening of the transition wavelength range and ultimately decreasesthe solar absorptance and increases the IR emittance.

FIGS. 3A and 3B illustrate the surface roughness of the absorbersubsequent to annealing. No significant surface roughness change uponsample annealing is observed, indicating that the annealing process doesnot significantly (e.g., to where it would be noticeable or negativelyimpact functionality) degrade the surface roughness. FIG. 3A is anatomic force microscopy (“AFM”) image of an S-SS solar absorber with a10 nm thick nickel layer before annealing and FIG. 3B is an AFM image ofthe S-SS solar absorber the 10 nm thick nickel layer after annealing atabout 600° C. for 7 days. The sample retains the groove structurecreated by the mechanical polishing process applied to the stainlesssteel substrate. The root mean square roughness (Rq) of the samplebefore and after annealing is calculated to be 6-8 nm using a NanoScopeAnalysis software.

FIGS. 4A-4D are AFM images illustrating the morphology change of asingle cermet layer before and after annealing. FIGS. 4A-4D are AFMimages of the morphology changes of a single cermet layer deposited on amechanically polished stainless steel substrate coated with a 10 nm Nilayer without any ARC layer after annealing at 600° C. for 7 days. FIG.4A illustrates the morphology of a cermet1 layer with a high metalvolume fraction in Al₂O₃ before annealing and FIG. 4B illustrates themorphology of the same sample after annealing. In an embodiment, a “highmetal volume fraction” refers to a metal volume fraction above about 62%and a “low metal volume fraction” refers to a metal volume fractionbelow about 56%. FIG. 4C illustrates the morphology of a cermet2 layerwith a low metal volume fraction in Al₂O₃ before annealing and FIG. 4Dillustrates the morphology of the same sample after annealing.

In another embodiment, two cermet samples (C1 and C2) were fabricatedwithout being disposed in contact with anti-reflective coating (“ARC”)layers, and were evaluated in terms of their phases and morphologybefore (“pristine”) and after annealing. The multilayer stacks depositedonto the stainless steel substrates consist of a 10 nm nickel bondinglayer and a single cermet layer with the only difference between the twosamples being the metal particle concentration in the cermet layers andtheir respective thicknesses (C1 and C2 as detailed in Table 1). Bothsamples C1 (FIG. 4B) and C2 (FIG. 4D) show significant changes in theirfilm morphology upon annealing. Similar to the previous sample (S-SS),the C1 and C2 samples start out with a groove surface structure;however, the annealing process leads to a rapid growth of the Ni—W alloywithin the cermet layer from diameters of about 80 nm to about 300 nmor, in some embodiments, about 400 nm. And the roughness increases fromabout 6-8 nm to about 47-50 nm. The difference in the metal volumefraction and layer thickness between sample C1 and C2 does not affectthe particle growth and roughness change. However, the unchangedroughness of the previous sample (S-SS) with double ARC and much thinnerdouble-cermet layer may indicate that the ARC layers suppress theparticle growth within the cermet or the particle growth is much lesspronounced in significantly thinner cermet layers.

FIG. 5 illustrates XRD patterns for pristine and annealed cermetcoatings. FIG. 5 illustrates the phase analysis before and afterannealing for cermet coatings with 10 nm nickel layers disposed onstainless steel for both cermet1 and cermet2 as noted, this phaseanalysis was conducted using X-ray diffraction and shows the sharp peaksfor the stainless steel substrate and the Ni—W alloy in thesingle-cermet layers. No diffraction peaks are observed for thedielectric Al₂O₃ even after annealing at 600° C. for 7 days due to itsstable amorphous nature. However, X-ray diffraction spectra show anadditional monoclinic FeWO₄ phase after sample annealing. Iron atomsdiffuse at high temperatures from the stainless steel substrate into thecermet layer and may react with tungsten and residual oxygen to form theobserved FeWO₄ phase.

FIG. 6 is a chart of Raman spectra of pristine and annealed cermetcoatings. In particular, FIG. 6 illustrates Raman measurements showingtwo distinct peaks located at 882 cm⁻¹ and 691 cm⁻¹ for the annealedsamples which can be traced back to A_(g) modes of FeWO₄. Also, thesolar absorber with thin nickel layer (S-SS) after annealing displays avery low reflectance in mid-IR range compared to that before annealingas shown in FIG. 2, which may indicate a destruction of IR reflector anda formation of nonmetallic phase between substrate and coatings. Thus,the degradation of the optical properties for the solar absorber sample(S-SS) may be the formation of FeWO₄ phase in the cermet layers at hightemperature.

FIG. 7 illustrates the spectral bidirectional reflectance response ofsolar absorbers fabricated according to certain embodiments of thepresent disclosure. The solar absorber samples (indicated by S—Ni/SS andS—W/SS in Table 1) were fabricated with 300 nm thick metal layers as thediffusion barrier between the stainless steel substrate and the doublecermet layer. Nickel and tungsten were employed as indicated as thediffusion barrier metals due to their high melting point and low IRemittance which improves the spectral selectivity of the solar absorbercompared to the previous sample S-SS with a very thin nickel layer. Boththick metal layers in the samples S—Ni/SS and S—W/SS significantlyincreased the spectral reflectance in the mid-IR range without alteringthe spectral response below 2.5 μm.

FIG. 8 illustrates XRD patterns of solar absorbers fabricated accordingto certain embodiments of the present disclosure. FIG. 8 illustratesthat the sample with the thick nickel layer (S—Ni/SS) shows two nickelpeaks which disappear after sample annealing, indicating that the nickelreacts with iron atoms from the SS substrate. The sample with a thicktungsten layer (S—W/SS) did not appear to be affected by the sampleannealing, thus demonstrating a stable tungsten layer which prevents theiron diffusion.

FIG. 9 illustrates the experimental set up and results for steady statecalorimetric measurements of samples fabricated according to certainembodiments of the present disclosure. FIG. 9 illustrates both the solarabsorptance and total hemispherical emittance of a fabricated solarabsorber (S—W/SS) was directly measured at elevated temperatures (up to500° C.) using simple steady state calorimetric methods. Samples wereattached to a heater assembly and suspended in a vacuum chamber. Theelectrical heater power input employed was directly related to theradiation heat loss from the sample surface. Thus, the totalhemispherical emittance can be calculated with the electrical heaterpower inputs and the measured sample and surrounding temperatures. Thesolar absorptance was measured at elevated temperatures using a solarsimulator. The sample/heater assembly is suspended in the vacuum chamberfacing a viewport allowing the solar simulator beam to irradiate thesample surface. The solar absorptance can be obtained by varying theincident radiation power onto the sample and measuring the correspondingelectric heater power adjustments to maintain the sample surface at aconstant temperature. The near normal solar absorptance and totalbidirectional emittance are calculated from the spectral reflectancedata, indicating that the developed spectrally selective solar absorberwith tungsten infrared reflector layer can be a good candidate for hightemperature solar thermal applications (See Table 2 below).

TABLE 2 Before Annealing After Annealing Sample Absorptance EmittanceAbsorptance Emittance S-SS  91.7% 8.63% 90.66% 15.99% S-Ni/SS 93.20%5.46% 91.38% 14.10% S-W/SS  92.2% 5.65% 90.77%  5.7%

The near-normal solar absorptance (divergence half angle of about 15°)is close to independent of temperature with a value of about 0.9 whichis in good agreement with the calculated solar absorptance from thespectral data. It has been theoretically shown that cermet-based solarabsorbers exhibit a solar absorptance with only weak angle dependence.Thus, only little deviation from here demonstrated solar absorptanceshould be expected even for concentrated solar power applications with alarge range of incident angles. However, future research efforts couldexperimentally investigate the angle dependence of the solar absorptanceto quantify the effect. The total hemispherical emittance shows thetypical temperature dependence of a spectrally selective solar absorberwith approximately 0.09 at 100° C. and 0.15 at 500° C.

FIG. 10 illustrates the spectral properties of a plurality of solarabsorbers fabricated according to embodiments of the present disclosure.The tungsten metal layer thickness may be optimized to keep theproduction cost minimal without losing the low emittance and long termthermal stability of the solar absorber. A plurality of solar absorberswith tungsten layer thicknesses of 10, 50, 100, and 200 nm werefabricated as indicated in Table 1 and their spectral properties werecompared before and after the annealing at 600° C. for 7 days. Thecurves in FIG. 10 of wavelength v. % reflectance are in the followingorder, and the corresponding compositions/configurations are listed inorder below in Table 3, which comprises the same values for eachcomposition/configuration as Table 1.

TABLE 3 Ordered Results from FIG. 10 Bonding layer/IR C1 C2 Substratelayer thickness thickness ARC1 ARC2 Sample material thickness/type (nm)(nm) (nm) (nm) S-W/SS-3 SS 100 nm W 11 28 25 55 S-W/SS-2 SS 200 nm W 1128 25 55 S-W/SS-3 (annealed) SS 100 nm W 11 28 25 55 S-W/SS-4 SS  50 nmW 11 28 25 55 S-W/SS-2 (annealed) SS 200 nm W 11 28 25 55 S-W/SS-4(annealed) SS  50 nm W 11 28 25 55 S-W/SS-5 SS  10 nm W 11 28 25 55S-W/SS-5 (annealed) SS  10 nm W 11 28 25 55

For the pristine (as-made, prior to annealing if annealing is performed)samples, the tungsten layer thickness only affects the spectralreflectance at wavelength larger about 2 The annealing process, however,alters the spectral response in the complete wavelength range with thelargest effect at wavelengths longer than about 1.2 μm. The spectralreflectance increases and the thermal stability improves with increasingtungsten layer thickness. A tungsten layer thickness of 100 nm (as inexamples S—W/SS-3) is sufficient to provide good (commercially scalableand usable) thermal stability and to act as a low emittance coating onstainless steel at high temperatures.

CONCLUSIONS

Iron atoms diffusing from the stainless steel substrate into the cermetlayer may not have a desirable effect on the optical properties of aselective solar absorber. The spectrally selective solar absorbersfabricated according to certain embodiments of the present disclosuremay be based on double cermet layers (W—Ni—Al₂O₃ cermet) with doubleantireflection layers on a mechanically polished stainless substratefabricated according to embodiments of the present disclosure. In someembodiments, a 100 nm thick tungsten layer may be employed to suppressthe degradation of the optical properties at high temperatures and tolower the emittance relative to the stainless steel substrate, whichimproves the spectral selectivity of the solar absorber, for example, inapplications where Ni may not be as effective an Fe-diffusion barrierand IR reflector. Using the materials, apparatus, systems and methodsdiscussed herein, a solar absorber was fabricated with a solarabsorptance of about 0.9 and total hemispherical emittance of about 0.15at an operating temperature of 500° C. In alternate embodiments, thislayer may comprise Tantalum (Ta), Titanium (Ti), Molybdenum (Mo),Chromium (Cr), Vanadium (V), Niobium (Nb), Zirconium (Zr), orcombinations thereof.

Exemplary embodiments are specifically disclosed and variations,combinations, and/or modifications of the embodiments and/or features ofthe embodiments made by a person having ordinary skill in the art arewithin the scope of the disclosure. Alternative embodiments that resultfrom combining, integrating, and/or omitting features of the embodimentsare also within the scope of the disclosure. Where numerical ranges orlimitations are expressly stated, such express ranges or limitationsshould be understood to include iterative ranges or limitations of likemagnitude falling within the expressly stated ranges or limitations(e.g., from about 1 to about 10 includes, 2, 3, 4, etc.; greater than0.10 includes 0.11, 0.12, 0.13, etc.). For example, whenever a numericalrange with a lower limit, R_(l), and an upper limit, R_(u), isdisclosed, any number falling within the range is specificallydisclosed. In particular, the following numbers within the range arespecifically disclosed: R=R₁+k*(R_(u)−R_(l)), wherein k is a variableranging from 1 percent to 100 percent with a 1 percent increment, i.e.,k is 1 percent, 2 percent, 3 percent, 4 percent, 5 percent, . . . , 50percent, 51 percent, 52 percent, . . . , 95 percent, 96 percent, 97percent, 98 percent, 99 percent, or 100 percent. Moreover, any numericalrange defined by two R numbers as defined in the above is alsospecifically disclosed. Use of broader terms such as “comprises,”“includes,” and “having” should be understood to provide support fornarrower terms such as “consisting of,” “consisting essentially of,” and“comprised substantially of.” Accordingly, the scope of protection isnot limited by the description set out above but is defined by theclaims that follow, that scope including all equivalents of the subjectmatter of the claims. Each and every claim is incorporated into thespecification as further disclosure, and each claim is an exemplaryembodiment of the present invention.

While exemplary embodiments of the invention have been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the scope or teachings herein. The embodimentsdescribed herein are exemplary only and are not limiting. Manyvariations and modifications of the compositions, systems, apparatus,and processes described herein are possible and are within the scope ofthe invention as claimed. Accordingly, the scope of protection is notlimited to the embodiments described herein, but is only limited by theclaims that follow, the scope of which shall include all equivalents ofthe subject matter of the claims. Unless expressly stated otherwise, thesteps in a method claim may be performed in any order. The recitation ofidentifiers such as (a), (b), (c) or (1), (2), (3) before steps in amethod claim are not intended to and do not specify a particular orderto the steps, but rather are used to simplify subsequent reference tosuch steps.

1. A method of fabricating a solar absorber, comprising: disposing afirst layer in contact with a substrate; disposing a second layer incontact with the first layer; disposing a third layer in contact withthe second layer; disposing a fourth layer in contact with the thirdlayer; and disposing a fifth layer in contact with the fourth layer,wherein disposing the fifth layer forms a solar absorber comprising anabsorbance within a first predetermined range and an emittance within asecond predetermined range.
 2. The method of claim 1, wherein thesubstrate comprises at least one of stainless steel, tantalum (Ta),titanium (Ti), copper (Cu), aluminum (Al), silicon (Si), quartz, andcombinations thereof.
 3. The method of claim 1, wherein the first layercomprises at least one of tungsten (W), tantalum (Ta), titanium (Ti),and combinations thereof, and wherein a thickness of the first layer isfrom about 10 nm to about 300 nm.
 4. The method of claim 1, wherein thefirst layer comprises Tungsten (W), Tantalum (Ta), Titanium (Ti),Molybdenum (Mo), Chromium (Cr), Vanadium (V), Niobium (Nb), Zirconium(Zr), or combinations thereof and wherein the thickness of the firstlayer is from about 100 nm to about 200 nm.
 5. The method of claim 1,wherein the second layer and the third layer each comprise at least oneof Nickel (Ni), Cobalt (Co), Iron (Fe), Tungsten (W), Tantalum (Ta),Titanium (Ti), Molybdenum (Mo), Chromium (Cr), Vanadium (V), Niobium(Nb), Zirconium (Zr), and Al₂O₃.
 6. The method of claim 1, wherein thethird layer has a lower metal volume fraction than the second layer. 7.The method of claim 1, wherein the fourth layer comprises at least oneof Al₂O₃, MgO, TiO₂, V₂O₃, ZrO, or combinations thereof.
 8. The methodof claim 1, wherein the fifth layer comprises SiO₂.
 9. The method ofclaim 1, wherein disposing the second layer on the first layer bonds thesecond layer to the substrate.
 10. A solar absorber comprising: areflector layer disposed in contact with a substrate; a first cermetlayer disposed in contact with the reflector layer; a second cermetlayer disposed in contact with the first cermet layer; and at least twoanti-reflective coating (ARC) layers, wherein at least one ARC layer isdisposed in contact with the second cermet layer.
 11. The solar absorberof claim 10, wherein the substrate comprises at least one of stainlesssteel, tantalum, titanium, copper, aluminum, silicon, quartz, andcombinations thereof.
 12. The solar absorber of claim 10, wherein thereflector layer comprises at least one of tungsten (W), tantalum (Ta),titanium (Ti), and combinations thereof, and wherein a thickness of thereflector layer is from about 10 nm to about 300 nm.
 13. The solarabsorber of claim 10, wherein the first cermet layer and the secondcermet layer each comprise at least one of Nickel (Ni), Cobalt (Co),Iron (Fe), Tungsten (W), Tantalum (Ta), Titanium (Ti), Molybdenum (Mo),Chromium (Cr), Vanadium (V), Niobium (Nb), Zirconium (Zr), and Al₂O₃,and wherein the second cermet layer has a lower metal volume fractionthan the first cermet layer.
 14. The solar absorber of claim 10, whereinthe first layer of the at least two ARC layers comprises at least one ofAl₂O₃, MgO, TiO₂, V₂O₃, ZrO, or combinations thereof.
 15. The solarabsorber of claim 10, wherein the second layer of the at least two ARClayers comprises SiO₂.
 16. The solar absorber of claim 10, wherein thereflector layer is a bonding layer between the substrate and the firstcermet layer.
 17. The solar absorber of claim 10, wherein the reflectorlayer comprises a plurality of separately deposited reflector layers.18. A solar absorber comprising: a reflector layer disposed in contactwith a substrate; a first cermet layer disposed in contact with thereflector layer, wherein the reflector layer comprises at least one oftungsten (W) or nickel (Ni).
 19. The solar absorber of claim 17, furthercomprising a second cermet layer disposed in contact with the firstcermet layer and at least two anti-reflection (ARC) layers, wherein atleast one ARC layer is disposed in contact with the second cermet layer.20. The solar absorber of claim 17, wherein the substrate comprises atleast one of stainless steel, tantalum, titanium, copper, aluminum,silicon, quartz, and combinations thereof.
 21. The solar absorber ofclaim 17, wherein a thickness of the first cermet layer is from about 10nm to about 300 nm.
 22. The solar absorber of claim 17, wherein thefirst cermet layer and the second cermet layer each comprise at leastone of Nickel (Ni), Cobalt (Co), Iron (Fe), Tungsten (W), Tantalum (Ta),Titanium (Ti), Molybdenum (Mo), Chromium (Cr), Vanadium (V), Niobium(Nb), Zirconium (Zr), and Al₂O₃, wherein the second cermet layercomprises a lower metal volume fraction than the second layer.
 23. Thesolar absorber of claim 17, wherein the first layer of the at least twoARC layers comprises at least one of Al₂O₃, MgO, TiO₂, V₂O₃, ZrO, orcombinations thereof.
 24. The solar absorber of claim 17, wherein thesecond layer of the at least two ARC layers comprises SiO₂.
 25. Thesolar absorber of claim 17, wherein the reflector layer bonds thesubstrate and the first cermet layer.